This application claims the Paris convention priority of Japanese Patent Application No. 11-27934/1999 filed on Feb. 4, 1999, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery, and more particularly, it relates to improvement of an active material for the purpose of providing a lithium secondary battery exhibiting good charge-discharge cycle performance.
As a positive electrode active material of a lithium secondary battery, MoO3 (molybdenum trioxide) and Nb2O5 (diniobium pentoxide) have been proposed (T. Tsumura, Solid State Ionic, Vol. 104, p. 183 (1997) and Japanese Laid-Open Patent Publication No. 59-90359/1984).
Although MoO3 belongs to the orthorhombic system and is a comparatively stable oxide among molybdenum oxides, the charge-discharge cycle performance of a lithium secondary battery using MoO3 as a positive electrode active material is not good. This is because the crystal structure of MoO3 is largely changed through repeated expansion and shrinkage during charge and discharge. Also, Nb2O5 belongs to the orthorhombic system and is the most stable oxide among niobium oxides, but the charge-discharge cycle performance of a lithium secondary battery using Nb2O5 as a positive electrode active material is not good. This is because the crystal structure of Nb2O5 degrades in a small number of cycles through repeated expansion and shrinkage during charge and discharge.
Accordingly, an object of the invention is providing a lithium secondary battery exhibiting better charge-discharge cycle performance than the lithium secondary battery using MoO3 or Nb2O5 as a positive electrode active material.
One lithium secondary battery (first battery) of this invention comprises a positive electrode, a negative electrode and a nonaqueous electrolyte, and one of the positive electrode and the negative electrode includes, as an active material, an orthorhombic composite oxide represented by a composition formula, MxMo1xe2x88x92xOy (wherein M is at least one transition element selected from the group consisting of Cu, V, Mn, Fe, Co and Ni; 0 less than xxe2x89xa60.46; and 2.6xe2x89xa6yxe2x89xa63.1), or an orthorhombic lithium-containing composite oxide obtained by incorporating lithium into the orthorhombic composite oxide.
Another lithium secondary battery (second battery) of this invention comprises a positive electrode, a negative electrode and a nonaqueous electrolyte, and the positive electrode includes, as an active material, an orthorhombic composite oxide represented by a composition formula, MxNb2xe2x88x92xOy (wherein M is at least one transition element selected from the group consisting of V, Cr, Mo, W. Mn and Fe; 0 less than xxe2x89xa60.6; and 4.7xe2x89xa6yxe2x89xa65.3), or an orthorhombic lithium-containing composite oxide obtained by incorporating lithium into the orthorhombic composite oxide.
As a result, the invention provides a lithium secondary battery exhibiting better charge-discharge cycle performance than a lithium secondary battery using MoO3 or Nb2O5 as a positive electrode active material.